gbm: Support dynamically loading named backends

gbm: Support dynamically loading named backends

If the user specifies a backend name explicitly
via an environment variable and it is not in the
list of built-in backends, attempt to load it at
runtime.

runtime-loaded backends get a new gbm_backend_desc
struct instance for each device using them (A
small increase in memory usage to eliminate the
need for the locking and bookkeeping sharing them
would require), so these structures need to be
freed when destroying devices using runtime-loaded
backends.

Signed-off-by: James Jones <jajones@nvidia.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9902>